1. Field of the Invention
The present invention relates to an automatic focusing circuit and more particularly, to an improvement of an automatic focusing circuit in a video camera for automatically matching the focus in response to a video signal obtained from an image sensor.
2. Description of the Prior Art
Conventionally, in an automatic focusing apparatus for a video camera an approach utilizing a video signal itself obtained from an image sensor for evaluating the state where the focus is controlled has been developed. According to such an approach, many good characteristics can be obtained. For example, there exists essentially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be exactly matched. Furthermore, according to this method, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very simple as a mechanism.
The automatic focusing circuit according to the above described method controls a focusing motor basically utilizing a control method referred to as a hill-climbing servo system. Such a hill-climbing servo system is disclosed in, for example, U.S. Pat. No. 4,638,364, U.S. Pat. No. 4,614,975, Japanese Patent Laying-Open Gazette No. 58505/1983 and No. 103776/1985. Briefly stated, a high frequency component of the video signal obtained from the image sensor is detected every one field as a focus evaluating value, the detected current focus evaluating value is always compared with a focus evaluating value detected one field before, and a focusing ring continues to be slightly vibrated so that the newest focus evaluating value may always take the maximal value, that is, maintain the in-focus state.
Furthermore, Japanese Utility Model Laying-Open Gazette No. 135712/1985 disloses a system for detecting the maximal value where the focus evaluating value is changed from an increasing tendency to a decreasing tendency while driving the focusing ring in a one-way direction, determining that the position corresponding to the maximal value is the in-focus position, and returning the focusing ring to the position and stopping the same.
As described in the foregoing, according to the conventional automatic focusing technique, control is performed such that the focus evaluating value in the current field is directly compared with the focus evaluating value one field before and the focusing ring is inverted or stopped if the focus evaluating value in the current field is decreased. Therefore, when the camera and the object are at rest in actually using the video camera to record an image, good automatic focusing operation can be expected.
However, when either of the camera itself or the object moves, the focus evaluating value varies irrespective of the degree of matching the focus. Particularly, in the case of so-called "unintentional movement of a camera" caused by unintentional movement of the hands and so-called "unintentional movement of an object" caused by abrupt movement of the object, there occurs the following problem. More specifically, "unintentional movement" occurs since an image of the object in the imaging surface of the image sensor moves within an exposure time (1/60 sec. or 1/30 sec. in the normal video camera). As a result, fine patterns of the object are blurred and the high frequency component of the video signal is decreased, so that it is impossible to distinguish the object similarly to the defocused state. Thus, when such "unintentional movement" occurs, the focus evaluating value is temporarily and largely decreased irrespective of whether the in-focus state is actually achieved or not. Therefore, according to the above described conventional automatic focusing technique, such temporary decrease in the focus evaluating value is determined as the defocused state, so that unnecessary rotation of the focusing ring is occurred or the focusing ring is stopped in the position other than the true maximal value, that is, the in-focus position. Such wrong motions of focusing ring of this kind causes a great difficulty in actually recording an image. In addition, in the above described method for stopping the focusing ring in the position corresponding to the maximal point, the focus evaluating value may be decreased due to the above described unintentional movement in spite of the in-focus state. In such a case, such decrease in the focus evaluating value is determined as the defocused state, so that driving of the focusing ring is resumed, resulting in unclear picture due to such unnecessary movement.